GT Kit Jetting

[simon k]

running a gt206 with 25mm dell and clubman.105 main jet, d22 third clip from the top,AQ266 atomiser.modified airbox
previously had a couple of light seize's when touring and rolling off the throttle into a very lean mid range, dropping the clip cure this.

[Diablo]

Done right they can make 17hp on our dyno with a JL3

What's that Al, 206cc?
Are you still running yours?

No its still a 186. Haven't used it for a while but the motor is still fine. I think the work best with a std stroke crank. Mine runs a 30mm but I did a better one recently with a 25mm.

[soulsurfer]

Been playing with the jetting the last week trying to get rid of a hot spot (700 deg on EGT) at 3/8 throttle doing 1 mile 3rd gear runs at around 6-6.5k rpm. After settling on a 102 main jet, I fitted a D26 needle in the hope that it would at least get a little boggy, no, I then upped the atomisor from AQ266 to a 288, no difference. Next was a D24 which as least got me onto some richness, but still have the hot spot according to my EGT guage The plug was looking a gunmetal colour on a NGK B8ES, so I changed to a B7 and it's just starting to get a brown colour but the hot spot is still there

I've not had a chance to do a proper plug chops in 4th gear as the local roads aren't lending themseves and will need to travel to do it, plus I'm not confident in the jetting yet, but should I be worried by this 3rd gear run or not or should I be looking for a problem elsewhere

[Diablo]

I'd have a re check of everything Mike. These kits are usually a piece of p155 to jet in. Usual needle is D22 with AQ266 atomisor and anything from a 98 up to a 110 main jet depending on pipe and filter arrangement.

[soulsurfer]

I checked everything and couldn't see any thing obvious so stripped the top end down to have a look, compression ratio is around 9:1 and squish is 1.3mm. Any comments welcome on the clinder head colour or black piston crown etc...




This is all I could find, would this be the cause of the problem?

[soulsurfer]

FFS sealed inlet, deglaze cylinder, new rings all fitted up, exactly the same problem. Checked timing, it was around 16-17 deg, reset to 19. No difference. AQ268, D29 needle, should be f**king unrideable
Pissed off, wife's pissed off, the baby's pissed off, well
and truly in the doghouse and the tennis elbow is playing up. Great

HEELLLLPP!!!!

[oldbiker]

saw your post and picture of the head on the four letter web site (****) I cant post there,
but the question that struck me was what happened to the squish band??
did it fall of in the machine shop??

I would ask MB to put a squish band of 40-50% back on the head,

I`m not a Lambretta expert but understand that a compression ratio of about 9:1 or 9.5:1 maximum is the norm

[soulsurfer]

saw your post and picture of the head on the four letter web site (****) I cant post there,
but the question that struck me was what happened to the squish band??
did it fall of in the machine shop??

I would ask MB to put a squish band of 40-50% back on the head,

I`m not a Lambretta expert but understand that a compression ratio of about 9:1 or 9.5:1 maximum is the norm

This one...

[Rich_T]

I'm affraid it is not possible to magically go to the machine shop and order up 40% squish band just like that. The vast majority of heads are all mild modifications of a standard Lambretta head because this is what is economicaly available to all.

If you did machine a head with 40% squish band or more you would find yourself having to machine deeper into the head to get the squish volume and then you run into three critical problems which are far more relivant:
1. You run the danger of breaking through the head entirely
2. The spark plug is now well and truely out of position, too low in the chamber.
3. The spark plug needs a spacer because the larger chamber leaves the plug protruding.

Then there is the who debate about squish bands in anycase. Some modern engines don't have them and some do, it is my belief you only need them when you are upping compression significantly. From our global Lambretta point of view, that is pretty much a standard side squish head and has been this way since circa 1958, provided the compression ratio is not over done it will be fine.

[soulsurfer]

That is just gasket sealant you can see, the head is machined flat.